In recent years, electronic communications devices have become increasingly smaller, and operating frequencies for electronic communications have become higher. As a result, it is desirable to provide effective electromagnetic wave shielding for electronic devices, so that an electronic device does not emit in excess of a permissible amount of electromagnetic interference (EMI), and does not receive external emissions of electromagnetic waves from another device. In addition, it is desirable that electromagnetic waves generated within an electronic device should be absorbed so as to prevent interference resulting from internally scattered electromagnetic waves. Furthermore, as wireless and mobile devices become increasingly pervasive, the necessity of suppressing EMI using an absorption material affixed to a surface, such as the wall in a building or a room, becomes increasing important to prevent reflection and interference of an electromagnetic wave at the surface.
Generally, it is desirable that an EMI shielding material, or an electromagnetic compatibility (EMC) material have, in addition to good EMI shielding performance, flexibility to facilitate mounting. In addition, it is desirable for the shielding material to be processed into a sheet for ease of handling and mounting. From such a perspective, an EMI shielding material including a material generating a conductive loss, a dielectric loss or a magnetic loss, such as metal and magnetic materials, dispersed in a resin material, may desirably be used as a shielding material. Furthermore, in order to increase EMI shielding performance, it is desirable that the shielding material include a high loading of fine filler particles in a resin material. However, the process of preparing a resin material with high filler loading of fine particles dispersed in a resin is not easy, and may also result in EMI shielding materials with reduced flexibility in some cases. Therefore, for the case of an EMI shielding sheet made using conventional resins and particulate material, it has been difficult to satisfy both the high flexibility and high EMI shielding requirements at the same time.
On the other hand, as described in Japanese Unexamined Patent Publication (Kokai) No. 64-52302 and Japanese Unexamined Patent Publication (Kokai) No. 2006-73991, there exists a technology for dispersing fine particles in a liquid medium having high dielectric constant, a liquid having an electric polarity, or an electrolyte solution, but not in a viscous resin. With respect to suitable solvent and electrolyte solutions, exemplary liquids include water, alcohols such as glycerin, methyl alcohol or ethanol, and aqueous halide solution such as aqueous sodium chloride solution or aqueous sodium iodide solution. These liquids may exhibit reduced efficacy in absorption of EMIs due to dielectric losses in the liquid, and some of the liquids may also be volatile and/or flammable. However, in processing these known materials into a sheet, it is desired to impart durability, flame resistance and heat resistance. In addition to these deficiencies, with respect to dispersion of fine particles in these liquids, only ferrite-based magnetic powders have been generally employed.
There are also examples wherein an ionic liquid is used as an EMI absorbing material. PCT Published Patent Application No. WO2006/053083 describes an EMI absorbing material wherein a polymerizable ionic liquid monomer is solidified by polymerization. PCT Published Patent Application No. WO2004/069327 describes an EMI shielding structure comprising two transparent window glass plates and an ionic liquid sealed between the transparent window glass plates. In such cases, EMI shielding was attempted using an ionic liquid, however, it is difficult to achieve adequate EMI shielding using only an ionic liquid. Furthermore, it has not previously been known to use fine particles of EMI absorbing materials in these ionic liquid materials, and it may be difficult to obtain a high loading of filler in solid materials due to their structure.